LU-2449 osd: osd-zfs to handle ./.. properly

ZFS does not store directory records . and .. on a disk.
Instead they are generated upon request (like in zfs_readdir()).
with the patch osd-zfs does the same for namespace directories.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: I910f9bad807c0e5a2bee1fd481470576ceaf4444
Reviewed-on: http://review.whamcloud.com/4990
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>